Knob and Tube (K&T) wiring was an early standardized method of electrical wiring which was extremely prevalent in the 1920’s and 30’s. While prevalent in the early part of the 20th century K&T was installed in some locations as late as the 1970’s. It was known that other methods such as better insulated wiring was safer and more effective but K&T could be installed for less.
There are several ways of detecting K&T wiring. One sign is that the circuit will have no electrical ground path. Also where ever the wire passes through lumber ceramic tubes are installed to insulate the wire from the wood. Ceramic knobs, which are displayed in the image, are also used as an insulator when the wiring is surface mounted in a building. The illustration displays the way K&T wiring is usually connected to homes. The best place to look for these signs is where ever the homes wiring is visible such as in the basement or in the attic rafters. Another sign that your home may have K&T wiring is the presence of old push button switches and two pronged outlets with no ground.
K&T wiring is insulated with a rubberized cloth. This insulation is prone to damage and was designed to be suspended in the air. The insulation breaks easily and deteriorates when over heated by modern appliances which expose the wire to damage. The insulation is also highly flammable. I have provided a link below that goes in to greater detail about the insulation of K&T wiring.
<——- Damaged insulation and wiring in contact with other material.